The present invention relates generally to diagnostic imaging and, more particularly, to a method and apparatus of acquiring imaging data at more than one energy range using multi-energy high speed switching filters.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
Recently, dual energy CT scanning commonly referred to as “tomochemistry” has increasingly been used as a means of gaining diagnostic information of a subject. A principle objective of dual energy scanning is to obtain diagnostic CT images that enhance contrast separation within the image by utilizing two scans at different chromatic energy states. A number of techniques have been proposed to achieve dual energy scanning including a “Two Crystal” method and a “Two kV” method. These two techniques were discussed by F. Kelcz, et al. in an article in Medical Physics 6(5), Sep./Oct. (1979) entitled “Noise Considerations in Dual Energy CT Scanning”. With respect to the “Two kV” technique, high frequency generators have made it possible to switch the kVp potential of the high frequency electromagnetic energy projection source on alternating views. As a result, data for two dual energy images may be obtained in a temporarily interleaved fashion rather than two separate scans made several seconds apart as required with previous CT technology. Simply scanning at two kVp potentials in an interleaved manner is not desirable as filtration of the dual energy levels remains a concern. For example, dual energy CT scanning with fixed filtration results in a dramatic decrease in signal strength when comparing the 80 kVp spectrum to the 140 kVp spectrum. Furthermore, the effective energy separation between the two spectrums is approximately 25 kV. Selectively filtering each kVp spectrum with different x-ray filtration can increase the energy separation to 45 kV in this case. This dramatically improves the effectiveness of dual energy CT imaging.
Therefore, it would be desirable to design an apparatus and method for acquiring imaging data at more than one energy state during a single scan without jeopardizing signal strength.